1. Field of the Invention
The present invention relates to the technical field of machines allowing homogenization of the thickness of fibrous sheets and/or spreading of such fibrous sheets, in order to obtain lower basis weights. In particular, the invention relates to a method and to a machine allowing homogenization of the thickness of such sheets, as well as to fabrics which may be obtained by applying such a method.
2. Description of Related Art
In the field of composite materials, the applicant was interested in proposing textile fabric sheets having a thickness as homogenous as possible, so as to obtain parts with controlled final mechanical properties. In the case of fabrics, conventionally consisting of an interlacing of warp yarns and of weft yarns, the latter is particularly difficult.
The reinforcements for a composite are exclusively used with addition of resin with different methods. The geometry of the final composite part therefore directly results from the thicknesses of the reinforcement used. It is then clear that the use of thinner reinforcements will provide lighter composite parts and also more performing since they have their fibres better oriented with less ripples. A fact which is less obvious but also true is that these reinforcements, being also used in a sometimes significant stack, it is necessary to reduce to a minimum their variations in thickness in order to make the geometry of the obtained composite part more reliable and robust. As the individual variabilities of the folds will gradually add up, a great variability in thickness of the reinforcement will inevitably cause a strong variability in thickness in the final part during the use of methods such as vacuum infusion.
Various documents are interested in spreading of fabrics, without however mentioning the impact which may have the spreading applied on the thickness and in particular on the thickness deviations which have the obtained spread textile sheets. Mention may be made of documents U.S. Pat. No. 4,932,107, U.S. Pat. No. 5,732,748, EP 670 921, WO2005/095689 and WO 94/12708. It is important to note that a tissue does not leave a weaving machine with homogenous thickness and openness factor on its width. Indeed, the actual principle of weaving induces a shrinkage phenomenon well known to one skilled in the art. This shrinkage is a reduction in the width of the warp sheet before and after weaving. It is due to the interlacing action of the warp and weft yarns. The latter cover a shorter final distance because of their ripples over and under the warp yarns. The result of this is a reduction in the width of the sheet upon leaving the comb of a weaving machine. As this shrinkage is related to the ripples of the weft yarns, it is not homogenous over the width of the fabric by the fact that the weft yarns are more free, close to the edges and less held by less numerous neighbouring warp yarns. As they are less blocked and more free, these edge of yarns therefore ripple more, the result of this is then a larger thickness and generally a larger openness factor. The thickness difference between the edges and the medium increase with the basis weight of the fabric.
It should also be noted that the over-thickness phenomenon of the edges is very locally enhanced by the use of generally thermoplastic selvage yarns used on the edges of the fabric for blocking the last warp yarns.
All the fabrics proposed in the prior art, which are spread out after their weaving, because of the applied spreading technique necessarily have significant thickness variation. In particular, in document U.S. Pat. No. 4,932,107, no mention of any width of the fabric, of the average width of the warp and weft yarns after spreading and of homogeneity of the openness factor on the fabric. Now, all these elements determine the more or less homogenous thickness of the fabric obtained after spreading. If the examples proposed in this patent are considered, if a tension of 200 g/cm is applied on a fabric with a width of 1.5 m, the value of the tension on the roller will be 150×200=30,000 i.e. 30,000 g. This value is sufficient for generating flexure of the rollers preventing the obtaining of a parallelism between the axes of the rollers and therefore a homogeneous pressure on the fabric, because of a higher pressure on the edges. There results a limitation of the width of the fabric to be processed in connection with the diameter of the rollers and of their length. In order to attempt to circumvent this difficulty, an increase in the diameter of the rollers may be contemplated for limiting flexure, but in this case, the inertia of the latter will then become significant and the energy required for obtaining the amplitude and the frequency will increase in proportion. Moreover, it may be noted that patent U.S. Pat. No. 4,932,107 applied in its example 3B, 2 rollers with the diameter of 125 mm with a single upper vibrating roller with a diameter of 60 mm, which on the one hand does not give the possibility of obtaining satisfactory spreading and on the other hand homogenization of the thickness. In a more general way, all the techniques for spreading fabrics described in the prior art do not give the possibility of adapting to the initial differences in thickness which the fabric has and therefore do not give the possibility of obtaining satisfactory spreading and homogenization of the thickness.
There also exist fabrics made in two steps, the first step being the formation of sheets with low basis weight consolidated via a polymeric binder, and then producing the interlacing for forming a fabric. Such fabrics because of the preliminary consolidation of the sheets provide lesser possibilities in terms of deformability during their applications. Further, the polymeric binders used may not be compatible with the sheet of requirements under hygrothermal stress of the final composite part.